Interlock vs Transfer Switch: Which Generator Connection to Use

Both solve the same problem legally: making it impossible for utility power and generator power to be connected at the same time. They do it differently, and for a portable generator the interlock is usually the better value.

How does an interlock work?

A breaker interlock kit is a metal plate that mounts on the panel's deadfront, over the main breaker and a dedicated generator breaker.

The plate slides. In one position it covers the generator breaker, so only the main can be on. Slide it the other way and it covers the main, so only the generator breaker can be on.

You physically cannot have both on. Not by mistake, not in the dark, not by forgetting a step — the plate is in the way.

It's a purely mechanical solution to a human-error problem, which is why it's accepted as equivalent protection to a transfer switch.

The full installation

An interlock alone isn't the whole job:

1. The interlock kit, listed for your specific panel model
2. A generator breaker in the panel, sized to the generator's output
3. A power inlet box on an exterior wall — a male inlet you plug the generator cord into, so no cord has exposed live pins
4. A feeder from the inlet box to the generator breaker
5. Permit and inspection

The inlet box and feeder are needed for a transfer switch too, so they're not a differentiator — the saving is in the switch itself and the subpanel wiring it would need.

Interlock kit or transfer switch — which is better?

Why the interlock often wins for portables

Cost. Meaningfully cheaper, and the difference is real money rather than a rounding error.

Whole panel access. You can power any circuit in the house — you just have to manage which ones are on. A subpanel transfer switch fixes your choices at installation time; the interlock leaves them open. If you decide next winter that you want the garage circuit, you just switch it on.

Flexibility. Priorities during a summer outage differ from a winter one. An interlock lets you change your mind.

Simplicity. Fewer components, nothing to fail.

Where it falls short

You are the load management. Before switching the generator breaker on, you switch off the circuits you don't want — electric water heater, range, dryer, air conditioning. Then bring loads up gradually.

Forget, and you'll overload the generator. It'll trip rather than break, but it's an inconvenience in the dark, and it's a discipline you have to maintain every time.

Manual only. No automatic operation. If the power fails while you're away, nothing happens. For a standby system that starts on its own, you need an ATS — that's not an interlock's job.

Panel compatibility. Kits are panel-model specific. Most mainstream panels have one; some older or unusual panels don't, and using a kit not listed for your panel voids its listing and will fail inspection.

Panel space. You need two adjacent free positions for the generator breaker, and the interlock has to fit the panel's physical layout. A full panel may need rearranging first — see how many circuits can a panel hold.

Choosing

An interlock suits you if:

• You have a portable generator
• You'll be home to operate it
• You want flexibility over which circuits you power
• A listed kit exists for your panel
• Cost matters

A manual transfer switch suits you if:

• You want the prioritization decided once and enforced automatically
• No listed interlock exists for your panel
• You'd rather not manage breakers during an outage
• Multiple people in the house may need to operate it

An automatic transfer switch is required if:

• You have a standby generator
• Outages while you're away must be covered
• Someone in the house depends on powered medical equipment
• You want load shedding so a smaller generator serves a larger house

See transfer switch explained and manual vs automatic transfer switch.

Installation is not DIY

Whichever you choose, installation means working inside an energized panel where the main lugs remain live with the main breaker off, and it's permitted, inspected work in essentially every jurisdiction.

The interlock kit itself looks simple enough that people attempt it. The plate is the easy part; the generator breaker, the feeder, the inlet box, and the neutral bonding decision are not. See generator neutral bonding and electrical work homeowners can legally do.

Standards and code reference

The standards behind this guide, for looking up in the edition your jurisdiction has adopted:

• NEC 702.5 — transfer equipment must prevent inadvertent interconnection
• NEC 110.3(B) — a listed interlock must be the one listed for that panel

Code editions and local amendments vary. Confirm the adopted edition with your AHJ, and treat manufacturer instructions as governing wherever they are more restrictive.

Where to go next

• How to connect a generator to your house
• Generator inlet box
• Backfeeding dangers
• Size it properly: generator sizing calculator

More in our generator and backup power guides.